To provide automobile engines with improved fuel efficiency and higher performance for environmental protection, recently, so-called downsizing which reduces engine displacement by 20-50% is accelerated, and direct-injection engines are combined with turbochargers to increase compression ratios. Improvement in the efficiently of engines inevitably results in higher engine temperatures, which may cause power-decreasing knocking. Accordingly, improvement in the coolability of parts particularly around the valves has become necessary.
As a means for improving coolability, JP 7-119421 A discloses a method for producing an engine valve comprising sealing metal sodium (Na) in a hollow portion of a hollow valve stem. JP 3-60895 A teaches a method for directly buildup-welding a valve seat on a cylinder head of an aluminum (Al) alloy by high-density heating energy such as laser beams, which is called “laser clad method.” An alloy for buildup-welding the valve seat is a dispersion-strengthened Cu-based alloy comprising boride and silicide particles of Fe—Ni dispersed in a copper (Cu)-based matrix, Sn and/or Zn being dissolved in primary Cu-based crystals.
The valve temperature during the operation of an engine is about 150° C. lower in the above sodium-filled valves (valve temperature: about 600° C.) than in solid valves, and the Cu-based alloy valve seats produced by the laser clad method lowers the solid valve temperature by about 50° C. (valve temperature: about 700° C.), preventing knocking. However, the sodium-filled (Na-enclosed) valves suffer a high production cost, so that they are not used widely except some vehicles. The Cu-based alloy valve seats produced by the laser clad method, which do not contain hard particles, have insufficient wear resistance, suffering seizure by impact wear. Also, the direct buildup-welding on cylinder heads needs the drastic change of cylinder head production lines and large facility investment.
To improve the thermal conductivity of a valve seat press-fit into a cylinder head, JP 10-184324 A discloses a two-layer structure comprising a valve-abutting layer containing Cu powder or Cu-containing powder (Cu content: 3-20%) and a valve seat body layer (Cu content: 5-25%), and JP 2004-124162 A discloses the infiltration of Cu or a Cu alloy into a sintered Fe-based alloy in which hard particles are dispersed.
JP 2001-500567 A discloses a sintered Cu-based alloy valve seat made of a dispersion-hardened Cu-based alloy having excellent thermal conductivity, in which hard particles are dispersed. Specifically, it teaches a starting material powder mixture comprising 50-90% by weight of main Cu-containing powder and 10-50% by weight of additional Mo-containing alloy powder, the Cu-containing powder being Cu powder hardened by dispersed Al2O3, and the Mo-containing alloy powder comprising 28-32% by weight of Mo, 9-11% by weight of Cr, and 2.5-3.5% by weight of Si, the balance being Co.
Though JP 2001-500567 A teaches that the Cu powder dispersion-hardened by Al2O3 can be produced by atomizing a Cu—Al alloy melt to Cu—Al alloy powder, and heat-treating the atomized Cu—Al alloy powder in an oxidizing atmosphere to selectively oxidize Al, it is actually difficult to increase the purity of a Cu matrix in which Al2O3 formed from an Al-dissolved Cu—Al alloy is dispersed.